Recent developments, challenges, and future research directions of tomographic characterization of fractured aquifers

This work reviews various studies of hydraulic and pneumatic tomography for estimation of flow properties of fractured geologic media with hydraulic and pneumatic tomography. The underlying conceptual inversion models can be broadly classified as continuum and discrete fracture network models and deterministic and stochastic approaches. A heterogeneous continuum method applies porous media parameters, while a DFN approach utilizes structural and hydraulic properties of fractures. An overview of field, laboratory, and synthetic studies with applications of hydraulic, pneumatic, or tracer tomography for the characterization of fractured geologic media shows that most studies rely on a heterogeneous continuum conceptual model and geostatistical methods to achieve a solution to the inverse problem. The application of a heterogeneous continuum model results in hydraulic properties that are representative of both fracture and matrix. Therefore, this approach may be more operationally useful for large scale sites with a non-negligible hydraulic conductivity of the rock matrix and high fracture intensity. The flow properties of single fractures can be estimated by applying a discrete fracture network (DFN) model. However, assumptions concerning fracture patterns and corresponding flow properties can lead to an oversimplified geological model. Possibilities for future research include integrating additional data and results from other inversion methods, the application of neural networks for inversion, the implementation of inversion results for the prediction and optimization of processes according to the planned application at the site, and opportunities for real-time inversion.